I would like to get a board for slalom racing in rought water conditions, for winds from solid 20knots up to 30-40.

I am 80kg, competing at national level, and I will use this board in full power 6.7 conditions down to 5.5.

Which board is easier in chop and gusty winds from the two above? Futura 93 is not an option for me because it is not registered at ISAF...

By the way, our "national" level racing is not as pro as PWA, and races in 30-40 knots are mostly won by guys who make good starts and jibes, top speed is not so important because the water is so rough that you are on the edge all the time. I would really like to have a board that is super easy and still fast enough!

I have sailed both, and when it comes to very rough water, the ST93 would be the board of choice. In those conditions the board isn't in contact with the water very often, so it evens up top speed. So the ST would then have the edge in my opinion with its control. The only thing to consider is the ST is 99L.

Actually, I prefer boards with more volume that ride "smaller" in those conditions.
The reason is that strong winds are always very gusty where we race and extra volume is very helpful in turns and in lulls.